One of the most conventional processes for producing a metal oxide phosphor is a solid phase method comprising the steps of mixing solid materials such as carbonates or oxides of metals as a raw material for a phosphor at a predetermined composition ratio with respect to the metal, and firing the mixture to yield a multi-element metal oxide. Since this method comprises the step of mixing the raw material powder in solid phase state, the mixture has obviously a non-uniform phase microscopically, and the composition shift is unavoidable, due to generation of a different phase. Further, since the particle size of the resultant metal oxide is relatively large and non-uniform, post-process such as crushing and screening is indispensable to obtain a metal oxide of small particles, resulting in the low yield of the metal oxide. Furthermore, the control of particle size is not so easy as expected.
An exemplary process for adjusting particle size is to add flux prior to firing. According to this process, particle size is adjusted primarily by selecting an appropriate kind and added amount of flux, and the firing temperature. According to such a method, however, control of particle size distribution is almost impossible, although the average particle size of the resultant phosphor is easily adjusted. Further, it is obvious that the metal composition of the metal oxide is inevitably deviated from what it is supposed to be because flux is added to the mixture. Accordingly, control with respect to the composition of the raw material components is cumbersome, and it is highly likely that an excessive component may be precipitated as a different phase.
It is effective to use a raw material of uniform state in order to make the starting material itself uniform for synthesizing. A liquid phase method based on a chemical process, as exemplified by sol-gel method or coprecipitation method, is known as such a method. According to these conventional liquid phase methods, however, even if the solution containing the starting material is uniform, it is inherently inevitable that the resultant substance after the synthesis fails to achieve uniformity in composition, even if attaining fine particulate state, because hydrolysis velocity, solubility product constant, etc. differ depending on the kind of metal compound, and the starting material has to go through hydrolysis, neutralization, precipitation or the like after the step of preparing the solution.
As a measure for solving the above problem, U.S. Pat. No. 3,330,697, and Japanese Unexamined Patent Publication No. 11-181419 has proposed a method comprising the steps of forming metal complexes by reacting metal ions with oxycarboxylic acids or polyamino chelating agent or the like in water, adding a polyol such as ethylene glycol as a cross-linking agent to the metal complexes for esterification polymerization to thereby yield gelatinous complex polymer, and thermally decomposing the complex polymer. In these prior art methods, however, it is highly likely that segregation may occur because metal may leave from the metal complex in the esterification polymerization. Further, a step of crushing fired substance obtained after firing the gelatinous complex polymer is indispensable in order to use the resultant complex polymer as a starting material. The above prior art method makes it difficult to adjust the particle size of the resultant composite oxide in a predetermined range, makes the production process cumbersome, and raises the production cost, which is not desirable in an economical aspect.
Various processes of synthesizing a starting material have been proposed other than the above-mentioned process. However, the conventional processes failed to contribute to practical use in industrial scale because synthesizing steps are complicated and production cost is high. Further, in the currently available process of producing metal oxide powder, it is hardly possible to desirably control the shape and particle size of the metal oxide powder.
Recently, a process for synthesizing ceramics from the metal chelate complex has been developed. This process has been evaluated because it is highly expected that this process can suppress non-uniformity of composition with respect to the metal of the metal oxide. However, an appropriate technique capable of uniformly mixing metal chelate complexes of solid phase in molecular level has not been established. Therefore, it is conceived that significance of using metal chelate complexes has not been sufficiently verified.
In view of the above, it is an object of the present invention to provide a process for industrially advantageously producing a metal oxide phosphor of single phase having substantially spherical shape and substantially uniform particle size, and an excellent luminance property at a remarkably lower temperature, as compared with a conventional process, by using, as a starting material, a raw material powder of substantially uniform particle size with substantially spherical particles and having uniform composition as a mixture in molecular level.